Coordinated multiple point (CoMP) is a key feature in the Long Term Evolution (LTE) advanced to further improve the coverage of high data rates, the cell-edge throughput and/or to increase system throughput, with the aim to meet 3rd Generation Partnership Project (3GPP) LTE advanced requirements on downlink. In the present context, the expression uplink denotes the communication from a user equipment (UE) to a base station, while the expression downlink denotes communication in the opposite direction, i.e., from the base station to the user equipment. Downlink CoMP transmission refers to a system where the transmission at multiple, geographically separated antenna sites is dynamically coordinated. The coordination can either be distributed, by means of direct communication between the different sites, or by means of a central coordinating node. Examples of coordinated transmission schemes include two main categories: Coordinated scheduling and/or beamforming and Joint processing/ transmission.
Coordinated Scheduling and/or Beamforming
It is characterized by that data to a single user equipment is simultaneously transmitted from multiple transmission points, e.g., to, coherently or non-coherently, improve the received signal quality and/or cancel actively interference for other user equipments, which puts higher requirements on the coordination links and the backhaul since the user data need to be made available at the multiple coordinated transmission points.
Coordinated scheduling: This is a most straight-forward approach of CoMP, meaning that user equipments are scheduled to be served by their attached cells in a manner such that the mutual interference among them within the cooperating set is minimized. This can to some extent be seen as an extension of the Inter-Cell Interference Coordination (ICIC) functionality present already in LTE Rel-8. Since it is a scheduling based solution, the physical layer transmission and reception of user data remain unchanged.
Coordinated beamforming: When the transmission points in the cooperating set are equipped with antenna arrays, a multi-antenna user equipment may receive multiple data streams, where the streams may be transmitted from multiple transmission points or a single transmission point in the cooperating set. In addition, Grid-of-Beams (GOB) based Space-Division Multiple Access (SDMA) and/or Multiuser beamforming received considerable attentions. Coordinated beamforming requires estimates of transmit channel correlation matrices for antenna arrays at all transmission points in the cooperating set, which can be obtained by means of uplink Sounding Reference Signals (SRS).
Joint Processing/Transmission
Coordinated transmission schemes comprising joint processing/transmission is characterized by that data to a single user equipment is simultaneously transmitted from multiple transmission points, e.g., to coherently or non-coherently improve the received signal quality and/or cancel actively interference for other user equipments, which puts higher requirements on the coordination links and the backhaul since the user data need to be made available at the multiple coordinated transmission points.
Joint processing/transmission is according to some embodiments carried out from several transmission points and pre-coded in a coherent fashion. In Time Division Duplex (TDD) systems, channel reciprocity between uplink and downlink can in principle be used to acquire this CSI at the network side, based on transmission of uplink SRS. Meanwhile, the differences due to the analogue receive/transmit chains need to be worked out. In Frequency Division Duplex (FDD) systems, the CSI about the downlink channels must be explicitly fed back from each user equipment to its coordination centre. The bit rate needed to support this feedback on the uplink increases significantly with some factors, e.g., user equipment's speed and frequency-domain channel selectivity, which is a very challenging problem.
Implementation of Downlink CoMP
For different categories mentioned above, different design is considered. In the case of coordinated scheduling, the actual downlink transmission can be carried out exactly as for LTE Rel-8 implying no specific impact to the radio-interface specification. In the case of coordinated beamforming with multi-stream downlink transmissions, multiple user equipment-specific reference signals (DRS) is needed and standardized to support since coordinated beamforming employs adaptive beam weights which are not derived from a codebook and multiple DRS are used for channel demodulation.
In the case of joint transmission, two alternatives are possible: joint transmission can be seen as originating from a set of cell-specific antenna ports, which may be associated with different cells. This can be seen as a generalization of LTE Rel-8 code-book-based multi-antenna transmission to antenna ports of more than one cell. In this case, user equipment channel estimation would rely on the cell-specific reference signal associated with each of the antenna ports involved in the joint transmission. However, the user equipment would need to know how the transmission is carried out, i.e., the exact set of antenna ports that are involved in the joint transmission and what transmission weight is applied at each antenna port.
For non-codebook based beamforming, each site in a cooperating set will need channel information based on uplink SRS to generate beamforming vector. The beamforming accuracy is directly related to downlink CoMP performance. However, in a realistic downlink CoMP system, beamforming accuracy will face some challenges.
Different link quality from the user equipment to different site in a cooperating set. In a typical case, the user equipment location in a CoMP system depends on CoMP scheduling strategy and geometric structure, which possibly results in different distance between the user equipment and different site. Furthermore, uplink power control and uplink timing advance are adjusted only according to one of sites, e.g., the nearest site. Different uplink transmission quality will impact on beamforming accuracy.
Therefore, from user equipment point of view there are two kinds of site in a cooperating set in terms of different uplink transmission quality: the one which is relatively far away from the user equipment, which may be referred to as a “Weak” site and the one which is relatively near to the user equipment which may be referred to as a “Strong” site.
Based on the analysis above, downlink CoMP performance is to some extent impacted and limited by the weak sites, which possibly result in poor beamforming accuracy. So, to improve the beamforming accuracy in the weak sites can improve overall downlink CoMP performance, e.g., reduced interference in multi-user beamforming system and increased signal power in single-user beamforming system.
There are different strategies for non-codebook based beamforming in Frequency Division Duplex (FDD) and Time Division Duplex (TDD). In TDD, channel reciprocity can be exploited to benefit short-term beamforming by means of the method of channel matrix decomposition. Whereas for FDD, this would imply that the information is limited to path loss and long-term fading due to no channel reciprocity in separate bands. However, one general way for both FDD and TDD is to utilize spatial information, e.g., direction of arrival (DoA), regardless of whether or not channel reciprocity can be exploited.
It is thus a problem to determine the location of a user equipment in relation to a base station, e.g., by determine direction of arrival or distance with an improved accuracy.